1. Technical Field
This invention relates to gas turbine engine rotor assemblies in general, and to apparatus for controlling vibrations in rotor stages in particular.
2. Background Information
The fan, compressor, and turbine sections of a gas turbine engine typically include a plurality of stator vane and rotor stages. The stator vane stages direct air flow (referred to hereafter as "core gas flow") in a direction favorable to downstream rotor stages. Each stator vane stage includes a plurality of stator vanes extending radially between inner and outer static radial platforms. Each rotor stage includes a plurality of rotor blades extending radially out from a rotatable disk. Depending upon where the rotor stage is within the engine, the rotor stage either extracts energy from, or adds energy to, the core gas flow. The velocity of the core gas flow passing through the engine increases with the rotational velocity of the rotors within the system. A velocity curve depicting core gas flow velocities immediately downstream of a stator vane stage reflects high velocity regions disposed downstream of, and aligned with the passages between stator vanes, and low velocity regions disposed downstream of, and aligned with each stator vane. The disparity between the high and low velocity regions increases as the velocity of the core gas flow increases. The high and low velocity regions have a significant effect on rotor blades passing through the region immediately downstream of the stator vanes.
Rotor blades typically have an areodynamic cross-section that enable them to act as a "lifting body". The term "lifting body" refers to a normal force applied to the airfoil by air traveling past the airfoil, from leading edge to trailing edge, that "lifts" the airfoil. The normal force is a function of: (1) the velocity of the gas passing by the airfoil; (2) the "angle of attack" of the airfoil relative to the direction of the gas flow; and (3) the surface area of the airfoil. The normal force is usually mathematically described as the integral of the pressure difference over the length of the airfoil. The difference in gas flow velocity exiting the stator vane stage creates differences in the normal force acting on the rotor blade.
The changes in normal force caused by the different velocity regions are significant because of the vibration they introduce into the rotor blades individually, and the rotor stage collectively. Low velocity regions can be described as producing a normal force on each rotor blade equal to "F", and high velocity regions described as producing a normal force on each blade "F+.DELTA.F", where .DELTA.F represents an additional amount of normal force. A blade rotating through the regions of low and high velocity gas flow will, therefore, experience periodic pulsations of increased force ".DELTA.F" (also referred to as a periodic excitation force). The frequency of the periodic excitation force is a function of the rotational speed of the rotor, since the number of stator vanes that create the low velocity regions is a constant. The magnitude of ".DELTA.F" depends upon the velocity of the core gas flow.
Vibrations in a rotor stage are never desirable, particularly when the frequency of the excitation force coincides with a natural frequency of the rotor stage; i.e., resonance. In most cases, resonance can be avoided by "tuning" the natural frequencies of the rotor stage outside the frequency of the excitation force by stiffening, adding mass, or the like. Alternatively, damping can be used to minimize the resonant response of the rotor stage. It is not always possible, however, to "tune" the natural frequencies of a rotor stage to avoid undesirable resonant responses. Nor is it always possible to effectively damp vibrations within a rotor stage. It would be a great advantage, therefore, to minimize or eliminate the cause of the vibration (i.e., the excitation force), rather than adapt the rotor stage to accommodate the vibration.